In recent years, the tensile strengths of a steel sheet have been increased to achieve increase in the reliability of automobile bodies and decrease in the weight of automobile bodies for the purpose of reduction in emissions. By using high strength steel sheets for automobile bodies, the same level of rigidity can be obtained with thickness and weight that are less than those of conventional steel sheets. However, it has been pointed out that high-tensile steel sheets have some problems. One of the problems is that the quality of weld zones in assembling an automobile body decreases as the strength of steel sheet increases.
As illustrated in FIG. 1, resistance spot welding is a method of obtaining a welded joint by clamping a sheet set 3 of two or more lapped steel sheets (in this case, a lower steel sheet 1 and an upper steel sheet 2) between a pair of upper and lower electrode tips (a lower electrode tip 4 and an upper electrode tip 5), pressing and welding the sheet set 3 so as to melt the sheet set 3, and forming a nugget 6 having a necessary size.
The quality of a joint obtained in this way is evaluated in terms of whether a sufficient nugget diameter is obtained, or by testing the tensile shear strength (the strength of the joint when a tensile test is performed by applying tension in a shearing direction of the joint), the cross tension strength (the strength of the joint when a tensile test is performed by apply tension in a peeling direction of the joint), the fatigue strength, and the like of the joint. Among these, the static strengths such as the tensile shear strength and the cross tension strength are very important as an index of the quality of a welded joint.
In particular, the tensile shear strength of a spot weld zone tends to increase as the tensile strength of a steel sheet increases. However, the cross tension strength increases only negligibly or may even decrease as the tensile strength of a steel sheet increases. It is considered that this is because the hardness of a weld zone and a heat-affected zone increases and thereby the toughness of these zones decreases, since the carbon equivalent Ceq of a high strength steel sheet, which is represented by the following equation (1) or the like, is inevitably large in order to achieve its high strength and since a welding is a process including rapid heating and rapid cooling.Ceq=C+ 1/24×Si+⅙×Mn(%)  (1)Here, “%” means mass %.
To ensure the joint strength when using a high strength steel sheet, the welding method may be improved by increasing the number of welding spots or by increasing the nugget diameter. However, increase in the number of welding spots would increase the influence of a shunt current, and also increase the operation time and therefore decrease the productivity. To increase the nugget diameter, it is necessary to increase the size of electrodes and increase the electrode force applied to the steel sheets to prevent spatter (expulsion) of a weld metal. These are constrained by the equipment and also have a disadvantage that the characteristics of a base metal may be impaired because the heat-affected zone is enlarged.
Therefore, various attempts have been made in order to ensure the strength of a welded joint with the number of spots and a nugget diameter that are equal to or less than those of existing methods. In particular, a large number of studies have been made regarding a tempering postheating method, which is a method in which a nugget and a HAZ are softened after making a weld solidified, quenched and then reheated. This method aims at increasing the strength of a welded joint by increasing the toughness of a nugget and reducing stress concentration in the vicinity of a weld.
Therefore, the quality certification of a joint has been often studied by focusing on the hardness. For example, in Patent Literatures 1 and 2, it is described that the hardness of a nugget and a heat-affected zone is measured and high strength of a joint portion can be certified if the measured hardness is in a certain range.